1. Field of Invention
The invention generally relates to lithography, and more particularly to support structures and arrangements for patterning devices.
2. Related Art
Lithography is widely recognized as a key process in manufacturing integrated circuits (ICs) as well as other devices and/or structures. A lithographic apparatus is a machine, used during lithography, which applies a desired pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device, which is alternatively referred to as a mask or a reticle, is typically used to generate a circuit pattern to be formed on an individual layer in an IC. This pattern is transferred onto the target portion (for example, comprising part of one, or several dies) of the substrate (for example, a silicon wafer). Typically, the pattern is transferred to a layer of radiation-sensitive material (for example, resist) provided on the substrate by imaging the pattern onto the radiation-sensitive material. A typical substrate may contain many such target portions that are adjacent to one another and are successively patterned.
Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
To increase production rate of scanned patterns, a patterning device, for example, a mask or reticle, is scanned at constant velocity, for example, 3 meters/second across a projection lens, back and forth along a scan direction. Therefore, starting from rest, the reticle quickly accelerates to reach the scan velocity, then at the end of the scan, it quickly decelerates to zero, reverses direction, and accelerates in the opposite direction to reach the scan velocity. The acceleration/deceleration rate is, for example, 15 times the acceleration of gravity. There is no inertial force on the patterning device during the constant velocity portion of the scan. However, the large inertial force encountered during the acceleration and deceleration portions of the scan, for example, approximately 60 Newtons (=0.4 kg of patterning device mass×150 m/sec2 of acceleration), can cause the patterning device to slip. Such slippage can result in a misaligned device pattern on a substrate.
Attempts to solve patterning device slippage include using a clamp, such as a vacuum system, to hold the patterning device in place and/or using a friction coating to increase friction between the patterning device and the clamp. However, ever increasing production rates demand ever faster direction reversals and, therefore, higher accelerations have reduced the benefits of these solutions. With clamps, the normal force between the patterning device and the clamp generates a friction force during the acceleration and deceleration portions of the scan. The friction force holds the patterning device in place during these portions. However, with vacuum clamps, the friction force is limited by the maximum differential pressure between atmosphere and the vacuum, which now is only about 1 bar. Further, the small surface area of patterning devices in contact with the clamps limits the normal force that can be generated by the clamps. Currently, the highest friction coefficient of suitable friction coatings is only approximately 0.25.